Designing and fabricating integrated circuits is a complex process that involves many discrete steps. A design specification is modeled logically, typically in a hardware design language (HDL). Software simulation and hardware emulation tools are used to verify the design logic. The design is converted into a technology specific netlist and is floorplanned and wired using physical design place and routing tools. Once the physical design has been completed and formatted correctly, the appropriate masks are created to fabricate the design as integrated circuits or a chip on a wafer.
Defects of various types occur during the chip manufacturing process. A defect may be simply a flaw in the chip caused by sources of imperfection inherent in the manufacturing process, or it may be caused by a systematic interaction between process and design. Failure analysis is often used to determine the root cause or defect mechanism, so that the manufacturing process or the design can be corrected to reduce yield loss.
In-line testing and inspection are performed during the chip manufacturing process with the goal of detecting defects and identifying the sources of these detected defects as close to real time as possible. A common type of in-line testing is a kerf test, which involves testing integrated circuit structures built in the kerf area between the chips on the wafer. If sufficient defective test structures are detected on a wafer, the entire wafer may be scraped or, alternatively, may be subjected to physical failure analysis for yield learning. One disadvantage of relying on kerf structures for yield learning is that these structures consume wafer surface area that would otherwise be available for chip fabrication. Other disadvantages of relying solely on kerf structures are that kerf structures may not have sufficient critical area to detect defects within test time constraints, and that kerf structures fail to contain as much layout design variety as product chips. Other vehicles for in-line testing include testsites and short loop wafers.
Wafer testing, also known as wafer final test, is performed after integrated circuits have been manufactured. Test patterns are applied to confirm that logic and memory elements have been fabricated correctly. Tests may be applied to confirm that specific functions operate correctly within specifications. Chip designs may include specific monitoring circuitry, such as wired out devices or on-chip performance screen ring oscillators. When all necessary tests pass for a specific chip on a wafer, the chip's spatial location on the wafer is stored for dispositioning. If a specific chip fails necessary testing and lacks sufficient redundancy to compensate for the fail, the chip is considered faulty and may be discarded. These chips represent the significant time and cost investment incurred in their manufacture. Additionally, these chips have captured defects, that if understood, could be used to improve yield.
Accordingly, there is a need for a method to design and build chip hardware that can be used to detect and localize defects in integrated circuits at the chip level.